1. Field of the Invention
The present general inventive concept relates to a memory device, and more particularly, to a variable resistance memory device.
2. Description of the Related Art
A semiconductor memory device may be classified into a volatile memory device and a nonvolatile memory device. The volatile memory device, for example, DRAM (dynamic random access memory) and SRAM (static random access memory), loses its stored data when its power supply is interrupted. The nonvolatile memory device, for example, PROM (programmable ROM), EPROM (erasable PROM), EEPROM (electrically EPROM) and flash memory device, retains its stored data even when its power supply is interrupted.
Also, next generation semiconductor memory devices, for example, FRAM (ferroelectric random access memory), MRAM (magnetic random access memory) and PRAM (phase change random access memory), are being developed according to a trend of high performance and low electric power of a semiconductor memory device. Material constituting next generation semiconductor memory devices has a characteristic that its resistance becomes different depending on the applied current or voltage and its resistance is maintained even when its power supply is interrupted.
Among the variable resistance memory devices, the phase change random access memory (PRAM) using phase change material typically has the fastest operation speed and has a structure that is advantageous in high integration.
The phase change memory device stores data using phase change material. The phase change material has two stable states (that is, an amorphous state and a crystalline state) that are different from each other. Since a conversion of the two states may reversibly occur, the phase change material can be converted from an amorphous state into a crystalline state and after that, can be converted into the original amorphous state. On the other hand, after the phase change material is converted from a crystalline state into an amorphous state, the phase change material can be converted into the original crystalline state. Resistivity of the phase change material in an amorphous state is higher than resistivity of the phase change material in a crystalline state. Data can be stored in a phase change memory cell, and the stored data can be read by using a difference of resistivity in accordance with a state of the phase change material.